It is a current practice in the production of silicon wafers for the fabrication of semiconductors to draw a monocrystalline elemental-silicon bar from a melt of elemental silicon contained in an upwardly open quartz crucible, a monocrystalline seed being lowered into the melt and being then drawn upwardly while the silicon within the crucible is maintained in a molten state by induction heating via a coil surrounding the crucible.
Because the heating effect is applied from the exterior inwardly and the nature of the process is such that, for the requisite temperature of the molten silicon in the crucible, the walls of the quartz crucible may be heated to higher temperatures close to the softening point of the material from which they are constituted, one problem which has been encountered is that of softening of the crucible walls. Another problem is the mutual attack of the silicon melt upon the quartz crucible and the material of the quartz crucible on the melt so that deterioration of the crucible can occur and the melt can be contaminated.
The problem is all the more complicated because, especially when the melt must be maintained at the aforementioned high temperature for prolonged periods, e.g. tens of hours, it is necessary to provide support for the crucible by means of a refractory material such as a carbon or graphite jacket which is intended to surround the crucible.
These problems have long been recognized and efforts to solve them have been made.
For example, in my commonly assigned copending application, Ser. No. 618,192, June 7, 1984, filed , and entitled, Method of and Apparatus for the Drawing of Bars of Monocrystalline Silicon, I have described a method in which mutual attack at an interface between the molten silicon and the quartz crucible is entirely avoided by isolating the silicon melt from the quartz crucible with the aid of a mass of elemental silicon granules from which the melt was formed. This system utilizes the fact that elemental silicon can act as a thermal insulator and a contamination barrier in the granular state.
Another approach which has been attempted and has been found to be successful is to apply a coating to the interior of the quartz crucible of a material more resistant to attack than the silicon dioxide of this crucible.
Suitable materials for this purpose include silicon carbide, silicon nitride, boron carbide and boron nitride.
Indeed, in the commonly assigned copending application, Ser. No. 614,434, filed May 25, 1984, (U.S. Pat. No. 4,505,948), and entitled, Method of Coating Ceramics and Quartz Crucibles with Material Electrically Transformed into a Wafer Phase, I have described a method in which the silicon carbide, for example, for coating the quartz crucibles is generated by a low-voltage, high-current arc struck between a silicon electrode and a carbon electrode by initially contacting these two electrodes and drawing them apart, in a vacuum of at least 10.sup.-5 torr.
That application describes a special case of my earlier work in the field of coating utilizing such low voltages and currents and described, for example, in my copending application Ser. No. 494,302, filed 13 May 1983 as a continuation-in-part of Ser. No. 358,186 of 15 Mar. 1982 (U.S. Pat. No. 4,438,183), in turn a continuation-in-part of Ser. No. 237,670, filed 24 Feb. 1981 (U.S. Pat. No. 4,351,855), incorporating the subject matter of Disclosure Documents Nos. 078,337, 078,334 and 078,329, all of 26 Feb. 1979 and Disclosure Document No. 082,283 of 5 July 1979.
While those systems have been found to be effective in generating the coating compounds and in applying them to a variety of substrates including the quartz crucibles in question here, in many cases the generation of a vapor phase of coating compounds or coating elements leaves much to be desired, especially when at least one of the materials adapted to form a coating or to be vaporized is of limited electrical conductivity.